1. Field of Invention
The invention relates to a current mirror and a transconductance amplifier containing the current mirror. In particular, the invention relates to a current mirror with a low static current and a transconductance amplifier containing the current mirror with a low static current.
2. Related Art
The current mirror is a basic and important analog device, widely used in bias circuits and the loads in an amplifier stage.
As shown in FIG. 1, a conventional current mirror has a first transistor M1 and a second transistor M2. The drain of M1 is coupled to its gate, and its source is coupled to the ground. The gate of M2 is coupled to the gate of M1, and its source is coupled to the ground. The first transistor M1 receives an input current Iin via the drain and generates a bias at the gate. The second transistor M2 generates an output current Iout at the drain. This current mirror is limited by small signal space and low output impedance.
As shown in FIG. 2, another conventional current mirror has in addition a third transistor M3 and a fourth transistor M4. The source of M3 is coupled to the drain of M1. The source of M4 is coupled to the drain of M2. The gates of the third transistor M3 and the fourth transistor M4 are coupled to a bias voltage Vbias. The gate of M2 and the gate of M1 are coupled to the drain of the third transistor M3, which receives an input current Iin. The fourth transistor M4 generates an output current Iout at its drain. This current mirror can increase the output impedance, but still suffers from the restriction of small signal space.
FIG. 3 shows yet another current mirror in the prior art. The drain of the first transistor M1 receives an input current Iin. The drain of the third transistor M3 receives a bias current Ibias. The current mirror only requires a voltage drop from the drain to the source in order to operate. Therefore, it has a larger signal space. However, it needs a continuous supply of the bias current Ibias, rendering a larger power loss.
FIG. 4 shows a transconductance amplifier in the prior art. It has a voltage input and a current output. Since the transconductance amplifier can provide a large current output, it is often used to drive a large capacity load CL to obtain a high slew rate. The output stage of the transconductance amplifier can be stage A or AB. The output stage A consumes a larger static current, resulting in unnecessary power loss. The output stage AB consumes a smaller static current. Nevertheless, it has a dead zone and no large slew rate can be produced with a small input voltage.
The U.S. Pat. No. 5,854,574 proposes a transconductance amplifier using three stages in series to increase the overall gain. It uses a push-pull output stage accompanied by a current mirror. Without any input signal, it only consumes a very small static current. When there is an input signal, a larger slew rate can be provided. However, this transconductance amplifier has four diode voltage drops between a fixed voltage VDD and the ground GND. Therefore, it cannot operate under a lower voltage supply.